Cellular Control

1
Cellular Control

• Unit 1
• Communication, Homeostasis and Energy

2
Meiosis

• Module 1 Cellular Control

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Learning outcomes

• describe, with the aid of diagrams and
  photographs,
• the behaviour of chromosomes during meiosis,
• the associated behaviour of the nuclear envelope,
  cell membrane and centrioles.
• (Names of the main stages are expected, but not
  the subdivisions of prophase)

4
Reproduction and variation

• Asexual reproduction
• Single organism divides by mitosis
• New organism is genetically identical to the
  parent
• Sexual reproduction
• Meiosis produces haploid gametes
• Which fuse at fertilisation to form a diploid
  zygote
• This produces genetic variation amongst offspring

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Human Life Cycle
Diploid Zygote 46
Mitosis
Haploid Sperm 23
Meiosis
Adult 46
Haploid Egg 23
fertilisation
6
Self assessment questions

• The fruit fly Drosophila melangaster has eight
  chromosomes in its body cells. How many
  chromosomes will there be in a Drosophila sperm?
• The symbol n is used to indicate the number of
  chromosomes in one set the haploid number of
  chromosomes. For example in humans n 23, in a
  horse n 32.
• How many chromosomes are there in a gamete of a
  horse?
• What is the diploid number of chromosomes (2n) of
  a horse?

7
Meiosis

• Meiosis is a reduction division
• Resulting daughter cells have half the original
  number of chromosomes
• Daughter cells are haploid
• Can be used for sexual reproduction
• Source of genetic variation
• Meiosis has two divisions
• meiosis I and meiosis II
• Each division has 4 stages
• Prophase, metaphase, anaphase, telophase

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Meiosis

• You can view an animation of Meiosis at
  http//www.cellsalive.com/meiosis.htm

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Meiosis I
Prophase I Chromatin condenses Homologous pairs form a bivalent Nucleolus disappears Spindle forms
Metaphase I Bivalents line up on equator of cell
Anaphase I Homologous chromosome in each bivalent are pulled to opposite poles
Telophase I Two new nuclear envelopes form Cell divides by cytokinesis
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Early Prophase 1
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Late Prophase 1
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Metaphase 1
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Anaphase 1
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Telophase 1
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Cytokinesis 1
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Meiosis II
Prophase II Nucleolus disappears Chromosomes condense Spindle forms
Metaphase II Chromosomes arrange themselves on equator Attach by centromere to spindle fibres
Anaphase II Centromeres divide Chromatids pulled apart to opposite poles
Telophase II nuclear envelopes reform around haploid nuclei Cell divides by cytokinesis
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Prophase II
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Metaphase II
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Anaphase II
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Telophase II
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Cytokinesis II
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Learning outcomes

• explain how meiosis and fertilisation can lead to
  variation through the independent assortment of
  alleles

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Key words

• Allele
• Locus
• Crossing over
• Maternal chromosome
• Paternal chromosome

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Alleles, locus and homologous chromosomes
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Meiosis and variation

• Meiosis enables sexual reproduction to occur by
  the production of haploid gametes.
• Sexual reproduction increases genetic variation
• Genetic variation increases the chances of
  evolution through natural selection

26
Meiosis and Variation

• Crossing over prophase I
• Independent assortment of chromosomes metaphase
  I
• Random assortment of chromatids metaphase II
• Random fertilisation
• Chromosome mutations
• Number of chromosomes
• Non-disjunction - polysomy or polyploidy
• Structure of chromosomes
• Inversion, deletion, translocation

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Crossing over
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During metaphase I
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During metaphase I
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No crossing over
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Crossing over new combinations of alleles
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Independent Assortment
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(No Transcript)
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Learning Outcomes

• explain the terms allele, locus, phenotype,
  genotype, dominant, codominant and recessive
• explain the terms linkage and crossing-over

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Glossary

• Gene
• Locus
• Allele
• Genotype
• Phenotype

• Heterozygous
• Homozygous
• Monohybrid cross
• Dominant allele
• Recessive allele

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Genetics

• Genetics is the study of inheritance
• Allele
• different varieties of the same gene
• Locus
• position of a gene on a chromosome

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Genetics

• Dominant
• An allele whose effect is expressed in the
  phenotype if one copy present
• Recessive
• An allele which only expresses as a homozygote
• Co-dominant
• Both alleles have an effect on the phenotype

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• Genotype
• genetic constitution of the organism
• Phenotype
• appearance of character resulting from inherited
  information

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• Homozygous
• Individual is true breeding
• Possesses two alleles of a gene e.g. RR or rr
• Heterozygous
• Two different alleles for a gene e.g. Rr

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Monohybrid inheritance

• Mendels First Law
• principle of segregation
• The alleles of a gene exist in pairs but when
  gametes are formed, the members of each pair pass
  into different gametes, thus each gamete contains
  only one of each allele.

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Inheritance of height in pea plants
gene Allele relationship Symbol
Height of pea plants Tall Dominant T
Height of pea plants dwarf recessive t

• Follow out the following cross to the F2
  generation
• Homozygous tall pea plant with a homozygous dwarf
  pea plant
• Write out the genotypic and phenotypic ratios
  from the F2 generation

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Inheritance of height in pea plants

• Laying out the cross
• P phenotype
• P genotype
• Gametes
• F1 genotype
• F1 phenotype
• F1 self-fertilised
• Gametes
• Random fertilisation
• F2 genotypic ratio
• F2 phenotypic ratio

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Pupil Activity

• Answer the questions on monohybrid inheritance
• Remember to write out each cross in full.

44
Cystic Fibrosis

• Cystic Fibrosis is caused by a mutation to a gene
  on one of the autosomes.
• Mutation
• Changes the shape of the transmembrane chloride
  ion channels (CFTR protein)
• The CFTR gene is found on Chromosome 7
• The faulty gene is recessive

45
Genetic Cross conventions

• Use symbols to represent two alleles
• Alleles of the same gene should be given the same
  letter
• Capital letter represents the dominant allele
• Small letter represents the recessive allele
• Choose letters where the capital and small letter
  look different
• The examiner needs to be in no doubt about what
  you have written

46
Inheritance of cystic fibrosis

• Three possible genotypes
• FF unaffected
• Ff unaffected
• ff cystic fibrosis
• Remember gametes can only contain one allele for
  the CFTR gene
• At fertilisation, any gamete from the father can
  fertilise any gamete from the mother
• This can be shown in a genetic diagram

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Genetic diagram showing the chances of a
heterozygous man and a heterozygous woman having
a child with cystic fibrosis.
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Phenotype ratio of offspring

• Genotype ratio 1FF2Ff1ff
• Phenotype ratio 3 unaffected1cystic fibrosis
• Can also be expressed as
• 25 chance of the child having cystic fibrosis
• Probability of 0.25 that a child will inherit the
  disease
• Probability that 1 in 4 that a child from these
  parents will have this disease.

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Learning Outcome

• Use genetic diagrams to solve problems involving
  sex-linkage and codominance.

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Sex-Linkage

• Sex-linked genes are genes whose loci are on the
  X or Y chromosomes
• The sex chromosomes are not homologous, as many
  genes present on the X are not present on the Y.
• Examples
• Haemophilia
• Fragile X syndrome
• Red green colour blindness

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Sex Chromosomes
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Factor VIII and Haemophilia

• Haemophilia is caused by a recessive allele of a
  gene that codes for a faulty version of the
  protein factor VIII
• XH normal allele
• Xh haemophilia allele

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possible genotypes and phenotypes
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Inheritance of Haemophilia
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Pedigree for a sex linked recessive disease
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Codominance

• Codominance describes a pair of alleles, neither
  of which is dominant over the other.
• This means both have an effect on the phenotype
  when present together in the genotype

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Codominance example

• Flower colour in plants
• CR red
• Cw white
• Genotypes
• CRCR red flowers
• CRCW pink flowers
• CWCW white flowers

• Write out a genetic cross between a pure breeding
  red plant and a pure breeding white plant.
• Carry out the cross to the F2 generation.
• Write out the genotype and phenotype ratio for
  the F2 generation

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Revision Question

• Coat colour in Galloway cattle is controlled by a
  gene with two alleles. The CR allele produces red
  hairs and therefore a red coat colour. The Cw
  allele produces white hairs.
• A farmer crossed a true-breeding, red-coated cow
  with a true-breeding white-coated bull. The calf
  produced had roan coat colouring (made up of an
  equal number of red and white hairs).
• Explain the result and draw a genetic diagram to
  predict the outcome of crossing two roan coloured
  animals.

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Inheritance of A, B, AB and O blood groups

• Human blood groups give an example of codominance
  and multiple alleles
• There are 3 alleles present
• IA
• IB
• Io

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• IA and IB are codominant
• Io is recessive
• Remember each human will only have two alleles

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Blood Groups
Genotype Phenotype
IAIA Blood Group A
IA Io Blood Group A
IAIB Blood Group AB
IBIB Blood Group B
IB Io Blood Group B
Io Io Blood Group o
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Inheritance of blood groups

• Carry out genetic crosses for the following
  examples
• Two parents have blood groups A and B, the father
  is IAIo and the mother is IBIo
• Father has blood group AB and the mother has
  blood group O
• Mother is homozygous blood group A and the father
  is heterozygous B.

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Learning Outcome

• Describe the interactions between loci
  (epistasis).
• Predict phenotypic ratios in problems involving
  epistasis.

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Dihybrid Inheritance

• Monohybrid cross
• Inheritance of one gene
• Dihybrid cross
• Inheritance of two genes

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Example dihybrid cross

• Tomato plants
• Stem colour
• A purple stem a green stem
• Leaf shape
• D cut leaves d potato leaves
• NOTE
• In the heterozygote AaDd due to independent
  assortment in meiosis there are 4 possible gamete
  combinations
• AD Ad aD ad

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Crosses

• Cross a heterozygous plant with a plant with a
  green stem and potato leaves
• Cross two heterozygous tomato plants

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Dihybrid Inheritance

• A woman with cystic fibrosis has blood group A
  (genotype IAIo). Her partner does not have
  cystic fibrosis and is not a carrier for it. He
  has blood group O.
• Write down the genotypes of these two people.
• With the help of a full and correctly laid out
  genetic diagram, determine the possible genotypes
  and phenotypes of any children that they may have.

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Autosomal linkage

• Each Chromosome carries a large number of linked
  genes
• If two genes are on the same chromosome then
  independent assortment can not take place.
• The genes are transmitted together and are said
  to be linked.

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Linked Genes

• Where linked genes are involved the offspring of
  a dihybrid cross will result in a 31 ratio
  instead of the 9331 ratio.
• Example
• In peas, the genes for plant height and seed
  colour are on the same chromosome (i.e. linked)

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Learning Outcome

• Describe the interactions between loci
  (epistasis).
• Predict phenotypic ratios in problems involving
  epistasis.

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Flower colour in sweet pea

• Flower colour
• Colourless precursor of a pigment C
• Gene that controls conversion of this pigment to
  purple P
• Both dominant alleles need to be present for the
  purple colour to develop
• Cross
• Cross two white flowered plants with the
  genotypes CCpp and ccPP
• Follow this cross through to the F2 generation

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Interactions of unlinked genes

• A single character maybe influenced by two or
  more unlinked genes.
• E.g. determination of comb shape in domestic
  poultry
• Dominant allele P pea comb
• Dominant allele R rose comb
• Two dominant alleles walnut comb
• No dominant alleles single comb

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Genetic Crosses

• Carry out a genetic cross between a true-breeding
  pea comb and a true breeding rose comb
• Follow this cross through to the F2 generation

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Inheritance of coat colour in mice

• Wild mice have a coat colour that is referred to
  as agouti.
• Agouti (A) is dominant to black (a)
• C is a dominant gene required for coat colour to
  develop
• A homozygous recessive cc means that no pigment
  can be formed and the individual is albino

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Inheritance of coat colour in mice

• Carry out a cross between a pure-breeding black
  mouse (aaCC) and an albino (AAcc)
• Follow this cross through to the F2 generation.

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Epistasis

• This is the interaction of different gene loci so
  that one gene locus masks or suppresses the
  expression of another gene locus.
• Genes can
• Work antagonistically resulting in masking
• Work complementary

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Epistasis ratios

• 9 3 4 ratio
• Suggests recessive epistasis
• 9 7 ratio
• Suggests epistasis by complementary action
• 12 3 1 ratio or 13 3 ratio
• Suggests dominant epistasis

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Predicting phenotypic ratios

• Read through pages 132 and 133 of your textbook
• Answer questions 1 7
• Complete the stretch and challenge question on
  eye colour in humans
• Read through and complete the worksheet provided
  for you on epistasis

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Learning outcome

• Use the chi-squared (?2) test to test the
  significance of the difference between observed
  and expected results.

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?2 (chi-squared) test

• Allows us to compare observed and expected
  results and decide if there is a significant
  difference between them.

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?2 (chi-squared) test

• Where
• S the sum of
• O observed value
• E expected value

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?2 (chi-squared) test

• Compare the ?2 value to a table of probabilities
• The probability that the differences between our
  expected and observed values are due to chance.
• If the ?2 value represents a probability of 0.05
  or larger, the differences are not significant
• If the ?2 value represents a probability of less
  than 0.05, it is likely that the results are not
  due to chance and there is a significant
  difference.

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Degrees of freedom

• The degrees of freedom takes into account the
  number of comparisons made.
• Degrees of freedom
• number of classes of data - 1

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Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Critical value 95 certain that the results are
not due to chance
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Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Accept null hypothesis There is no significant
difference, results have occurred due to chance
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Table of ?2 values
Degrees of freedom Probability greater than Probability greater than Probability greater than Probability greater than
Degrees of freedom 0.1 0.05 0.01 0.001
1 2.71 3.84 6.64 10.83
2 4.60 5.99 9.21 13.82
3 6.25 7.82 11.34 16.27
4 7.78 9.49 13.28 18.46
Reject null hypothesis accept experimental
hypothesis Difference is significant, not due to
chance
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Mammal question

• ?2 value 51.8
• Degrees of freedom 3
• Critical value (p0.05) 7.82
• Reject the null hypothesis
• There is a significant difference between
  observed and expected results
• Suggestions?
• The two genes are linked

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Variation

• What did you learn at AS level?

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Learning Outcomes

• Define the term variation.
• Discuss the fact that variation occurs within as
  well as between species.
• Describe the differences between continuous and
  discontinuous variation, using examples of a
  range of characteristics found in plants, animals
  and microorganisms.
• Explain both genetic and environmental causes of
  variation.

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Variation

• Variation is the differences that exist between
  individual organisms.
• Interspecific variation (between species)
• Differences that are used to assign individuals
  to different species
• Intraspecific variation (within a species)
• Individuals of the same species show variation
• Variation can be inherited or influenced by the
  environment.

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Types of variation

• There are two main types of variation
• Continuous variation
• Discontinuous variation
• There are two main causes of variation
• Genetic variation
• Environmental variation

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Continuous variation

• Existence of a range of types between two
  extremes
• Most individuals are close to a mean value
• Low numbers of individuals at the extremes
• Both genes and the environment interact in
  controlling the features
• Examples
• Height in humans
• Length of leaves on a bay tree
• Length of stalk of a toad stool

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Continuous variation

• Use a tally chart and plot results in a histogram

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Discontinuous variation

• 2 or more distinct categories with no
  intermediate values
• Examples
• Earlobes attached or unattached
• Blood groups A, B, AB or o
• Bacteria flagella or no flagella
• Flowers colour of petals
• Genetically determined
• The environment has little or no effect on
  discontinuous variation

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Discontinuous variation
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Causes of variation

• Genetic Variation
• Genes inherited from parents provide information
  used to define our characteristics
• Environmental Variation
• Gives differences in phenotype (appearance) but
  not passed on by parents to offspring
• Examples
• Skin colour tans with exposure to sunlight
• Plant height determined by where the seed lands

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Variation

• What you need to know for A2!!

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Learning outcomes

• Describe the differences between continuous and
  discontinuous variation.
• Explain the basis of continuous and discontinuous
  variation by reference to the number of genes
  which influence the variation.
• Explain that both genotype and environment
  contribute to phenotypic variation.
• Explain why variation is essential in selection.

99
variation

• Variation can be
• Discontinuous
• Each organism falls into one of a few clear-cut
  categories, no intermediate values
• Qualitative differences between phenotypes
• Continuous
• No definite categories
• A continuous range of values between two extremes
• Quantitative differences between phenotypes

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Genes and variation

• Discontinuous (qualitative) variation
• Monogenic inheritance
• Different alleles at same gene locus
• Different gene loci have different effects
• Epistasis, codominance, dominance and recessive
  patterns of inheritance

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Genes and Variation

• Continuous (quantitative) variation
• Polygenic inheritance
• Two or more genes
• Each gene has an additive effect
• Unlinked genes

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Polygenic Inheritance

• Example length of corn cobs
• Three genes A/a, B/b and C/c
• Each dominant allele adds 2 cm length
• Each recessive allele adds 1 cm length
• So
• AABBCC 12 cm long
• aabbcc 6 cm long
• Hmmm!!
• How long would AaBBCc be?
• How long would aaBbCc be?

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Genotype, environment and phenotype

• The environment can affect the expression of the
  genotype
• examples
• AABBCC has the genetic potential to produce cobs
  12cm long
• This could be affected by
• Lack of water, light or minerals
• Obesity in humans
• Affected by diet and exercise

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Genotype, environment and phenotype

• The environment influences the expression of
  polygenic traits more than monogenic traits.

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Learning Outcomes

• Use the HardyWeinberg principle to calculate
  allele frequencies in populations.

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Population genetics

• What is a population?
• Group of individuals of the same species that can
  interbreed
• Populations are dynamic
• The set of genetic information carried by a
  population is the gene pool.

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Allele Frequency

• To measure the frequency of an allele you need to
  know
• Mechanism of inheritance of that trait
• How many different alleles of the gene for that
  trait are in the population

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Hardy-Weinberg principle

• The Hardy-Weinberg principle is a fundamental
  concept of population genetics
• It makes the following assumptions
• Population is very large
• Random mating
• No selective advantage
• No mutation, migration or genetic drift

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The equations

• p frequency of the dominant allele
• q frequency of the recessive allele
• The frequency of the allele will be in the range
  0 1.
• 0 no one has the allele
• 0.5 half the population has the allele
• 1 only allele for that gene in the population

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Ok the equations

• Equation 1
• p q 1
• Equation 2
• p2 2pq q2 1
• Where
• p2 frequency of genotype DD
• 2pq frequency of genotype Dd
• q2 frequency of genotype dd

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Calculating the frequency of cystic fibrosis in
the population

• 1 in 3300 babies are born with cystic fibrosis
• All babies with cystic fibrosis have genotype nn
• Calculate q2
• Calculate q
• Calculate p
• Calculate frequency of genotype Nn
• If we have 30,000 people in our population how
  many will be carriers of the cystic fibrosis
  allele

112
Question

• Phenylketonuria, PKU, is a genetic disease caused
  by a recessive allele. About one in 15 000
  people in a population are born with PKU.
• Use the hardy-Weinberg equations to calculate the
  frequency of the PKU allele in the population.
• State the meaning of the symbols that you use,
  and show all your working.

113
The Answer

• Calculate q2 1 / 15000 0.000067
• Calculate q 0.0082

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Another question

• Explain why the Hardy-Weinberg principle does not
  need to be used to calculate the frequency of
  codominant alleles.

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Pupil Activity

• Answer the Hardy-Weinberg practice question.
• You have 10 minutes
• Starting NOW!!

116
The Answers

• q2 0.52 / q 0.72
• p 1 0.72 0.28
• p q 1 p2 2pq q2 1
• Answer 2pq / use of appropriate numbers
• Answer 40

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The other answers

• Any three from
• Small founder population / common ancestor
• Genetic isolation / small gene pool / no
  immigration /
• no migration / in-breeding
• High probability of mating with person having
  H-allele
• Reproduction occurs before symptoms of disease
  are apparent
• Genetic argument Hh x hh 50 / Hh x Hh 75
  affected offspring
• No survival / selective disadvantage

118
Learning Outcomes

• Explain, with examples, how environmental factors
  can act as stabilising or evolutionary forces of
  natural selection.
• Explain how genetic drift can cause large changes
  in small populations.

119
Variation and Natural Selection

• The set of alleles in a population is its gene
  pool
• Each individual can have any combination of
  alleles in the gene pool
• producing variation
• Some individuals more likely to survive
• They reproduce and pass genes on to offspring
• Advantageous alleles become more frequent in the
  population

120
Environmental Resistance

• Environmental factors that limit the growth of a
  population offer environmental resistance
• These factors can be biotic or abiotic

121
Selection pressures

• An environmental factor that selects for some
  members of a population over others
• Confers an advantage onto certain individuals

122
Stabilising Selection

• If the environment stays stable
• The same alleles will be selected for in
  successive generations
• Nothing changes, this is called stabilising
  selection

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Stabilising Selection
124
Stabilising Selection
125
Directional Selection

• Change in the environment resulting in a change
  in the selection pressures on the population
• Previously disadvantageous alleles maybe selected
  for
• Change in the genetically determined
  characteristics of subsequent generations of the
  species
• A.k.a. evolution

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Directional Selection
127
Directional Selection
128
Genetic Drift

• A change in the gene pool and characteristics
  within the population.
• This change has occurred by chance rather than as
  the result of natural selection.

129
Genetic Drift and Islands

• Genetic drift is thought to happen relatively
  frequently in populations on islands.
• Small populations
• Geographically separated from other members of
  their species
• Evidence
• Many isolated islands have their own endemic
  species of plants and animals

130
Genetic Drift

• Reduces genetic variation
• Reduce the ability of the population to survive
  in a new environment
• May contribute to the extinction of a population
  or species
• Could lead to the production of a new species

131
Genetic Drift Frog Hoppers

• The colours of the common frog-hopper are
  determined by seven different alleles of a single
  gene.
• The range of colours and their frequencies, on
  different islands in the Isles of Scilly, are
  very variable,
• There are different selection pressures on the
  different islands

132
Genetic Drift Frog Hoppers
133
The answers
134
Learning Outcomes

• Explain the role of isolating mechanisms in the
  evolution of new species, with reference to
  ecological (geographic), seasonal (temporal) and
  reproductive mechanisms.

135
Speciation

• Speciation is the formation of a new species.
• Species
• Group of organisms, with similar morphology and
  physiology, which can interbreed with one another
  to produce fertile offspring.

136
Speciation

• In the production of a new species, some
  individuals must
• Becomes morphologically or physiologically
  different from members of the original species
• No longer be able to breed with the members of
  the original species to produce fertile offspring.

137
Isolation

• Splitting apart of a splinter group
• Geographical isolation
• Organisms are separated by a physical barrier
• Reproductive isolation
• Two groups have become so different that they are
  no longer able to interbreed
• They are now a different species

138
Isolating Mechanisms

• Large populations may be split into sub-groups by
• Geographic barriers
• Ecological barriers
• Temporal barriers
• Reproductive barriers

139
Geographical Barriers (AS recap)

• Geographical barrier separates two populations of
  a species
• Two groups evolve along different lines
• Different selection pressures
• Genetic drift
• If barrier breaks down and two populations come
  together again, they may have changed so much
  that they can no longer interbreed
• They are now two different species

140
Isolating Mechanisms

• Speciation occurs when organisms live in the same
  place
• The barriers which can prevent two closely
  related species from interbreeding include
• Ecological
• Temporal
• Reproductive

141
Ecological Barriers

• Ecological barriers exist where two species live
  in the same area at the same time, but rarely
  meet.
• Example
• Two different species of crayfish, Orconectes
  virilis and orconectes immunis, both live in
  freshwater habitats in North America

142
Meet the Crayfish

• Orconectes virilis
• Not good at digging, cant survive summer drying
• Lives in streams and lake margins
• Orconectes immunis
• Lives in ponds and swamps,
• Can easily burrow into the mud when the pond
  dries up
• In streams and lake margins O. virillis is more
  aggressive and will drive O. immunis out of
  crevices where it tries to shelter

143
Temporal Barriers

• Two species live in the same place, and may even
  share the same habitat
• Do not interbreed as they are active at different
  times of the day, or reproduce at different times
  of year
• Example flowering shrubs in Western Australia

144
Meet the shrubs

• Banksia attenuata flowers in the summer
• Banksia menziesii flowers in the winter
• They can not interbreed

145
Reproductive barriers

• Even if species share the same habitat and are
  reproductively active at the same time, they may
  not be able to interbreed
• Different courtship behaviours
• Mechanical problems with mating
• Gamete incompatibility
• Zygote inviability
• Hybrid sterility

146
Meet the Mallards

• Different courtship behaviours
• A male mallard duck will only mate with a female
  who displays the correct courtship behaviour
• Although the pintail female looks similar to the
  Mallard female, her courtship behaviour will only
  attract a pintail male.

147
Learning Outcomes

• Explain the significance of the various concepts
  of the species, with reference to the biological
  species concept and the phylogenetic
  (cladistic/evolutionary) species concept.

148
The Species Concept

• In AS biology you defined a species as
• a group of organisms, with similar
  morphological, physiological, biochemical and
  behavioural features, which can interbreed to
  produce fertile offspring, and are reproductively
  isolated from other species

149
The two species concept

• Group of organisms
• Capable of interbreeding
• Capable of producing fertile offspring
• Reproductively isolated from other groups
• This is the Biological Species concept

• Group of organisms showing similarities in
  characteristics
• Morphological
• Physiological
• biochemical
• Ecological
• Behavioural
• This is the phylogenetic species concept

150
Biological Species concept

• Group of organisms that can interbreed and
  produce fertile offspring.
• Clear cut definition
• Limitation
• Can only be used for organisms that reproduce
  sexually

151
Phylogenetic species concept

• Also known as the
• Evolutionary species concept
• Cladistic species concept
• Different morphology between the two groups and
  certain that they evolved from a common ancestor
• Not rigorous but allows decisions to be made

152
Comparing the genetics

• Closely related organisms have similar molecular
  structures for DNA, RNA and proteins.
• Biologists can compare specific base sequences
  (haplotypes)
• The number of differences caused by base
  substitutions can be expressed as the divergence

153
Cladistics

• Clade
• Group of organisms with similar haplotypes
• In cladistic classification systems is assumes
  that the taxa are monophyletic, this means that
  it includes an ancestral organism and all its
  descendents.

154
Cladistic classification

• Focuses on evolution
• Places importance on using molecular analysis
• Uses DNA and RNA sequencing
• Uses computer programmes
• Makes no distinction between extinct and still
  existing species

155
Learning Outcomes

• Compare and contrast natural selection and
  artificial selection.
• Describe how artificial selection has been used
  to produce the modern dairy cow and to produce
  bread wheat (Triticum aestivum).

156
Selection

• Natural Selection
• Mechanism for evolution
• Organisms best adapted to their environments more
  likely to survive to reproductive age
• Favourable characteristics are passed on
• Produces organisms that are well adapted to their
  environment

157
Artificial Selection

• Humans select the favourable characteristics
• Humans allow those organisms to breed
• Produces populations that show one characteristic
  to an extreme
• Other characteristics retained may be
  disadvantageous

158
Artificial Selection and the modern dairy cow

• Breeds of cows with higher milk production have
  been artificially selected for
• Milk yield from each cow is measured and recorded
• Test progeny of bulls
• Elite cows given hormones to produce many eggs
• Eggs fertilised in vitro
• Embryos implanted into surrogate mothers
• A few elite cows produce more offspring than they
  would naturally

159
Disadvantage to high milk yields

• Health costs for artificially selected cows is
  higher due to
• Mastitis
• Ketosis and milk fever
• Lameness
• Respiratory problems

160
Artificial selection and bread wheat (Triticum
aestivum)

• Polyploidy
• Nuclei contain more than one diploid set of
  chromosomes
• Wild species of wheat have a diploid number (2n)
  of 14
• Modern bread wheat is hexaploid (6n), It has 42
  chromosomes in the nucleus of every cell

161
Getting from the ancestors to modern bread wheat
Wild einkorn AUAU 2n 14
Domestication and artificial selection
Wild Grass BB 2n 14
Einkorn AUAU 2n 14
x
162
Wild Grass BB 2n 14
Einkorn AUAU 2n 14
x
Sterile hybrid P AuB
Mutation that double chromosome number
Wild Grass DD 2n 14
Emmer Wheat AUAUBB 4n 28
x
163
Wild Grass DD 2n 14
Emmer Wheat AUAUBB 4n 28
x
Sterile hybrid Q AuBD
Mutation that double chromosome number
Common Wheat AUAUBBDD 6n 42
164
Continuing selection in wheat

• Breeders are continuing to try and improve wheat
  varieties
• Resistance to fungal infections
• High protein content
• Straw stiffness
• Resistance to lodging
• Increased yield